Surprisingly, the errors seems to be that a space was added between "IssueDate:" and "<=" which led to the error. Since by correcting this, the error does not seem to appear anymore, I assume this is solved. @Alex Putkov.Thanks for the help.

It's not a very common error. I don't think I've seen it before. Do you know what search request resulted in this error? Better yet do you have a replication procedure?

The error is returned by the following call:

CComVariant l_Emptyvar;
l_Emptyvar.vt = VT_ERROR; 
l_Emptyvar.scode = DISP_E_PARAMNOTFOUND; 
return MRSearchQuery->send(l_Emptyvar); //MRSearchQuery of type CComPtr<RSearchLib::IRSearchQuery>

It is actually random but lately the following generates the error:

• Rsearch request:
  
  • Asset class: "BOND"
  • Search Criteria: "Name: 'COOPERATIEVE RABOBANK UA' MaturityDate:2018-09-07=<=2019-10-04 IssueDate:<=2018-09-03"
  • Search Parameters: "NBROWS:100"

I'm afraid I do not reproduce the error on my end, which is not surprising if the error is indeed random. I assume the error message was constructed in your code by checking pIRSearchResponse parameter passed to OnUpdate event handler, right? What I would recommend as the next step in troubleshooting is to employ Fiddler or another HTTP analyzer to see what happens to the HTTP requests to the search service. For this particular search expression you should see the following HTTP request in Fiddler:

GET https://emea1.views.cp.thomsonreuters.com/search/ThomsonReuters/MarketsSearch/Services/2012/05/01/Search.svc/SearchGovCorpInstruments?query=%27Name(COOPERATIEVE%20RABOBANK%20UA)%27&$select=RIC,ISIN&$filter=MaturityDate%20gt%20datetime%272018-09-07%27%20and%20MaturityDate%20le%20datetime%272019-10-04%27%20and%20(IssueDate%20lt%20datetime%272018-09-03%27)&$top=100 

My expectation is that the failed request will be shown with HTTP status code other than 200. You may or may not get any insight into why the Search Web service did not fulfill the request. Either way, if indeed it's the Search Web service that fails to fulfill the request, the most likely solution is to check pIRSearchResponse in OnUpdate event handler (which I assume you're already doing), and resend the search request unless pIRSearchResponse.Status is either zero or 0x80040223. The latter corresponds to pIRSearchResponse.StatusDescription saying "No results were found matching your criteria". Please checkout previous discussion on this thread.